________________________________________________________________Wave Propagation
decreases its density) has the effect of increasing the velocity of sound
through the medium.
1-69. The velocity of sound in an elastic medium is expressed by the formula:
v=
E
d
1-70. Even though solids such as steel and glass are far denser than air, their
elasticities are so much greater that the velocities of sound in them are 15
times greater than the velocity of sound in air. Using elasticity as a rough
indication of the speed of sound in a given medium, we can state as a general
rule that sound travels faster in harder materials (such as steel), slower in
liquids, and slowest in gases. Density has the opposite effect on the velocity of
sound, that is, with other factors constant, a denser material (such as lead)
passes sound more slowly.
1-71. At a given temperature and atmospheric pressure, all sound waves
travel in air at the same speed. Thus the velocity that sound will travel
through air at 32F (0C) is 1,087 feet per second. But for practical purposes,
the speed of sound in air may be considered as 1,100 feet per second. Table
1-1 gives a comparison of the velocity of sound in various mediums.
Table 1-1. Comparison of Velocity of Sound in Various Mediums
TEMPERATURE
VELOCITY
MEDIUM
F
C
(Ft/Sec)
Air
32
0
1,087
Air
68
20
1,127
Aluminum
68
20
16,700
Carbon Dioxide
32
0
856
Fresh Water
32
0
4,629
Fresh Water
68
20
4,805
32
0
4,219
Lead
32
20
4,030
Salt Water
32
0
4,800
Salt Water
68
20
4,953
Steel
32
0
16,410
Steel
68
20
16,850
1-72. The science of sound is called acoustics. This subject could fill volumes of
technical books, but we scratch only the surface in this chapter. We present the
important points that you need for a better understanding of sound waves.
Acoustics, like sound, relates to the sense of hearing. It also deals with the
production, control, transmission, reception, and effects of sound. For the
present, we are concerned only with the last relationship--the effects of sound.
These same effects will be used throughout your study of wave propagation.
1-21
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